In a large number of applications employing carbon nanotubes, it is necessary to localize a catalyst in structures having small dimensions, such as, for example, vias, in order to produce interconnections having small diameters.
It is a matter in this case of catalyst nanoparticles, that is to say particles having their greatest dimension less than or equal to approximately 100 nm.
Furthermore, once the catalyst is localized in the hole, it is necessary for the process for growth of carbon nanotubes to be able to be applied in the hole.
Several techniques are used to date to deposit the catalyst, or any other nanoparticle, without selective localization between the upper layer and the bottom of the vias, for example.
In these cases, the nanoparticles are deposited over the entire surface and are subsequently removed by chemical mechanical polishing (CMP) or by ionic erosion of the upper layer in order to remain localized only in the bottom of vias. The deposition process must be directed in order to prevent the deposition of the nanoparticles and the growth of the carbon nanotubes on the sides of the holes.
Furthermore, one of the major difficulties is to obtain particles having small dimensions, of less than 50 nm, which are stable up to a temperature of approximately 1000° C. and in a sufficient density to obtain high densities of carbon nanotubes or (between 1011 and 1014 carbon nanotubes per cm2).
Many processes for the deposition of catalyst are described in the literature which are divided into two main categories: processes for the deposition of a thin film and processes for the direct deposition of nanoparticles.
With the processes for the deposition of a thin film, the thickness of the film controls the mean size, that is to say the statistical mean of the sizes of the particles, measured by scanning or transmission of electromicroscopy and statistical calculation and the density of the particles. However, the control of the size and the density of the particles cannot be carried out independently of one another.
The deposit can be annealed in order to obtain the nanoparticles by dewetting the film. The selectivity of the deposition on the desired regions is obtained by the combination of nonconforming depositions and etching.
This category includes PVD (physical vapor deposition) deposits, the deposits obtained by the chemical route starting from a solution, by reduction of metal salts, by decomposition of a metal salt or of an iron-storage protein (ferritin). In this case, there is no selectivity of the deposition, indeed even a difficulty in depositing the catalyst at the hole bottom because of problems of diffusion of the catalyst.
The electrochemical depositions of thin films on conductors are also included in this category of processes.
In the processes for the direct depositions of nanoparticles, the nanoparticles are produced either at the time of carrying out the deposition by laser decomposition of a salt of a catalyst, for example of ironpentacarbonyl, by plasma, by use of laser ablation of a target under vacuum, or, finally, by generation of aerosol introduced directly into a laminar flow reactor, or the nanoparticles are produced with a first process and then dissolved, and the deposition takes place subsequently on the surfaces.
In this case, there is no selectivity of the deposition with respect to the structure on which it is carried out. On the other hand, it is possible to use all the resources of chemistry in order to obtain nanoparticles which are well graded in diameter and thus with very narrow size distribution (standard deviation of the order of 0.2 nm).
In point of fact, in a great number of applications, it is necessary to carry out a selective deposition of nanoparticles in devices having small dimensions.
In general, these nanoparticles are catalyst nanoparticles.
In particular, in a great number of applications employing carbon nanotubes, it is necessary to localize a catalyst in structures having small dimensions, of the order of 100 nm or less, such as, for example, vias, in order to produce interconnections having small diameters.
The growth of the carbon nanotubes takes place starting from the catalyst nanoparticles.
“Nanoparticles” is understood to mean, in the invention, particles having a very low mean diameter of less than 10 nm, preferably between 2 and 4 nm.
In the case of the growth of carbon nanotubes, it is furthermore necessary that, once the catalyst is localized in the hole, the process for the growth of the carbon nanotubes be able to be applied in the hole.
The ideal characteristics for the deposition of nanoparticles, in particular the catalysts suitable for this application, are as follows:                particles having a very low diameter (nanoparticles),        particles which are stable with regard to temperature (in order to avoid coalescence and sintering of the individual particles). For this, it is preferable to use particles based on metal oxides rather than metal particles or layers,        selective deposition between the horizontal and vertical surfaces of vias,        selective deposition between the surfaces of the top and the bottom of the substrate in which the vias occur, and        control of the distribution of the particles (diameter φ, density D and standard deviation σ).        
The deposition of continuous films by PVD makes it possible to deposit particles of metal catalysts or of metal oxides with a selective deposition between the horizontal and vertical surfaces.
It also makes it possible to control the diameter of the particles.
However, it does not make possible selective deposition between the surfaces of the top and the bottom.
Neither does it make it possible to control the density of the particles and the standard deviation σ.
The deposition of continuous films of particles by electrochemical deposition makes possible the deposition of metal catalysts with selective deposition between the horizontal and vertical surfaces and also between the surfaces of the top and the bottom.
It also makes it possible to control the diameter of the particles.
However, it does not make it possible to deposit a metal oxide catalyst, which is insulating, and it does not make it possible to independently control the density of the particles, the size and the standard deviation σ.
The chemical deposition in solution of continuous films of particles makes possible the deposition of metal catalysts or of metal oxide catalysts and the control of the diameter of the particles.
However, it does not make possible a selective deposition between the horizontal and vertical surfaces and between the surfaces of the top and the bottom and does not make it possible to independently control the density of the particles, the size and the standard deviation.
The PVD deposition of nanoparticles makes possible the deposition of metal catalysts and metal oxide catalysts.
It also makes it possible to control the diameter of the particles, their densities and the standard deviation σ.
However, it does not make possible a selective deposition between the surfaces of the top and the bottom.
Thus, no current process meets all the criteria of an ideal deposition of catalyst.